A Wavelength-Division-Multiplexin, (WDM) cross-connect (a non-wavelength-changing one) is a device that can exchange any wavelength channel in any line with the same wavelength channel in any other line. The conventional design is to use a matrix of switches between sets of wavelength multiplexers, such as Waveguide Grating Routers (WGRs). For example see the publications
[1] M. K. Smit, "New focusing and dispersive planar component based on an optical phased array," Electron. Lett., vol. 24, pp. 385-386 (1988); PA1 [2] H. Takahashi, S. Suzuki, K. Kato and I. Nishi, "Arrayed-waveguide grating for wavelength division multi/demultiplexer with nanometer resolution," Electron. Lett., vol. 26, pp. 87-88 (1990); and PA1 [3] C. Dragone, "An N.times.N optical multiplexer using a planar arrangement of two star couplers," IEEE Photon. Technol. Lett., vol. 3, 812-814 (1991); each of which are incorporated herein by reference.
For 2 line.times.2 line versions, designs have been demonstrated with discrete components {e.g., see B. Glance, "Tunable add drop optical filter providing arbitrary channel arrangements," IEEE Photon. Technol. Lett., vol. 7, 1303-1305 (1995)} which is incorporated herein by reference, and with fully integrated components {e.g., see K. Okamoto, M. Okuno, A. Himeno, and Y. Ohmori, "16-channel optical add/drop multiplexer consisting of arrayed-waveguide gratings and double-gate switches," Electron. Lett., vol. 32, 1471-1472 (1996)}, also incorporated herein by reference.
Referring to the drawing, FIG. 1. is a top view of a prior-art waveguide array multiplexer 10 comprising a first array coupler 11 coupled via an array of n waveguides W.sub.1, W.sub.2, . . . , W.sub.n to a second array coupler 12. The array couplers 11 and 12 may be slab waveguide regions with a plurality of peripherally distributed waveguides. In one mode of operation, coupler 11 receives light at P.sub.i from an input waveguide 13, and coupler 12 distributes received light to a pair of output waveguides 14 and 15 near P.sub.0. The array of n waveguides between couplers 11 and 12 are arrayed in a "C" configuration with the waveguides spreading apart away from the couplers in order to reduce crosstalk. Each successive waveguide provides an optical path that is longer than its predecessor by a constant amount .DELTA.l.
In operation, light in the fundamental mode of input waveguide 13 spreads by diffraction from the center of coupler 11, forming a circular wavefront of constant phase. The waveguides around the circumference of coupler 11 are all excited with the same phase. Since the waveguides separate, the optical signals become decoupled. After decoupling, however, the light in the waveguides initially continues with a circular phase front and retains nearly the same amplitude distribution as when the waveguides were initially excited.
The array is designed such that each successive waveguide has an increased length .DELTA.l compared to its lower neighbor, and the waveguides terminate in a converging circle at coupler 12. If .DELTA.l=m.lambda..sub.o, where .lambda..sub.o is the wavelength in the medium and m is an integer order number, then the phase front of the light in coupler 12 forms a circular wave converging on the coupler axis at P.sub.o. At a different wavelength .lambda..sub.o +.DELTA..lambda., the phase front emerging from the array will be tilted at a small angle .theta., relative to the coupler axis CC' and will couple efficiently near P.sub.o to waveguide 15 which is angularly displaced by .theta. from the coupler axis. In this example, output waveguide 14 is positioned to couple to .lambda..sub.o and output waveguide 15 to .lambda..sub.o +.DELTA..lambda.. This device can thus be used to receive a two wavelength input from waveguide 13 (.lambda..sub.o and .lambda..sub.o +.DELTA..lambda.) and provide wavelength separated outputs .lambda..sub.o at 14 and .lambda..sub.o +.DELTA..lambda. at 15. The device thus acts as a grating and a demultiplexer. In the reverse direction, it can act as a multiplexer.
Many lightwave devices require an array of two or more waveguide paths of substantially equal length, such as dynamic wavelength equalizers disclosed by C. R. Doerr, in an article entitled "Proposed optical cross connect using a planar arrangement of waveguide grating routers and phase shifters," that appeared in IEEE Photon. Technol. Lett., vol. 10, pp. 528-530, 1998 (hereinafter the "Doerr Optical Cross Connect Article"); the wavelength cross connects disclosed by C. R. Doerr and C. Dragone, in another article entitled "Proposed optical cross connect using a planar arrangement of beam steerers," to appear in IEEE Photon. Technol. Lett., vol. 11, February, 1999; and optical cross connects such as that disclosed by C. R. Doerr, C. H. Joyner, and L. W. Stulz, in an article entitled "Integrated WDM dynamic power equalizer with potentially low insertion loss," IEEE Photon. Technol. Lett., vol. 10, pp. 1443-1445, 1998, all incorporated herein by reference.
FIG. 2 shows a schematic of a prior art device disclosed in U.S. Pat. No. 5,212,758 issued to Adar et al on May 18, 1993 for a "Planar Lens and Low Order Array Multiplexer", and incorporated herein by reference in its entirety. The device shown in FIG. 2. is similar to that of FIG. 1 except that the n waveguides (W.sub.1, W.sub.2, . . . , W.sub.n) between array couplers 21 and 22 are arrayed in an "S" configuration rather than a "C" configuration. Specifically, each waveguide comprises two substantially circular arcs which reverse direction of curvature at AA' half way between the couplers 21 and 22. Such "S" configurations have shown to be effective for making broadband multiplexers or planar lenses.
Notwithstanding these advances, there is a continuing need for optical devices that facilitate the transmission and management of optical signals.